We demonstrate the reversible rod-to-sphere shape transition of gold/mesoporous silica core/shell nanorods, where the shell acts as an elastic nanocontainer during the shape change. It is shown, that elongated core/shell nanorods are transformed into spherical core/shell particles at 300 degrees C. The anisometric shape of the composite particles can be recovered upon in-situ seeded growth of the gold core. The mesoporous silica shell acts as a nanoscale confinement, enabling control over the growth procedure during the chemical reaction. The shell of the particles was found to be elastic; it shows conformal shape-change with the core material during the heating and the subsequent seeded growth process. The effect of the reaction conditions during the seeded growth on the resulting particle morphology was also investigated. It is demonstrated, that depending on the growth conditions, core/shell nanorods or larger core/shell nanospheres can be obtained. The shape transformation cycle can be repeated for the same system several times, where the break-up of the confining shell represents the physical limit of the process. (c) 2013 Elsevier B.V. All rights reserved.